ADSORPTION AND DIFFUSION OF GUEST MOLECULES IN ZEOLITIC IMIDAZOLATE FRAMEWORK-90 BY COMPUTER SIMULATIONS

Abstract

Zeolitic imidazolate framework (ZIF)-90 has interested great consideration in gas separation such as CH4/H2, CH4/CO2, CH4/N2 due to its window size that is a factor for separation process. However, there is experimental evidence from adsorption and permeation studies that CH4 can enter the ZIF-90 framework despite the fact that the critical diameter of CH4 (3.8 Å) is larger than the window size of ZIF-90 (3.5 Å) assuming a rigid framework. Therefore, adsorption and diffusion of CH4 and H2 in the ZIF-90 were investigated by Molecular Dynamics (MD) and Gibbs Ensemble Monte Carlo. Various interaction force fields have been tested and a suitable one has been studied. Results of structural and dynamical properties of CH4 and H2 in ZIF-90 are presented in this work. The flexibility has the big influence to the self-diffusion coefficient (Ds) while the good adsorption isotherm can be produced in rigid model. The preferential adsorption site found in RDFs is the organic linker in both guest molecules CH4 and H2. Furthermore, gate opening effect in ZIF-90 was not found in this work when loading CH4 and H2 molecules up to a pressure of 260 bar. The results from the mixture CH4/H2 (1:1) show a promising potential of ZIF-90 for gas separation with 4.1 separation factor.